The present invention relates to a radio communication system such as a mobile radio telephone system which comprises a plurality of mobile units and a plurality of transmitting stations located in a plurality of radio zones constituting a service area. More particularly, the present invention relates to a radio signal transmission system or to a transmission diversity system wherein the plurality of stations transmit the same common signal (such as a calling signal) to a mobile unit by a plurality of carrier waves.
In a radio signal transmission system of this type, the stations generally use carrier waves of the same frequency to carry the same common signal in order to promote efficient frequency utilization. However, the carrier wave from each station often interferes with that from another station, producing a code error. This interference occurs in an "overlapping zone" between adjacent radio zones. In order to overcome such a problem, the following three transmission methods have been proposed.
In a first transmission method, as disclosed in U.S. Pat. No. 4,032,846 issued on June 28, 1977, the center frequency of the carrier waves at each of the plurality of transmitting stations are offset relative to each other. A drawback inherent in this method is that the carrier frequencies must be highly stable to maintain a constant offset.
A second transmission method proposed in the prior art employs an identical center frequency for each station but varies the modulation index at each station relative to each other. This is undesirable in that the S/N ratio of the signal is degraded in a zone of a lower modulation index except in the overlapping zone.
A third transmission method, as disclosed in U.S. Pat. No. 4,223,405 issued on September 16, 1980, used modulation signals whose waveforms differ from each other by a discrete amount. The disadvantage of this third transmission method is that both a wave shaping circuit (for producing the difference in waveform) and a wave combining circuit are necessary additions to the structure of the transmitters.